1. Technical Field
The present invention relates to surface-emitting type semiconductor lasers that emit laser light and methods for manufacturing the same.
2. Related Art
A surface-emitting type semiconductor laser is provided with a resonator formed in a direction orthogonal to a substrate surface, and emits laser light from the substrate surface. Compared to edge-emitting type semiconductor lasers that use parallel cleavage planes of a substrate as a resonator, surface-emitting type semiconductor lasers have various favorable characteristics. For example, surface-emitting type semiconductor lasers are suitable for mass-production, capable of direct modulation, and capable of operation with low threshold current, and a two-dimensional laser array structure can be readily formed with surface-emitting type semiconductor lasers.
A basic structure of a surface-emitting type semiconductor laser includes a lower mirror composed of a semiconductor multilayer film, an active layer and an upper mirror composed of a semiconductor multilayer film sequentially formed on a substrate. The lower mirror and the upper mirror form a resonator, and the active layer is disposed within the resonator. As the upper mirror and the lower mirror provided in the surface-emitting type semiconductor laser, distributed Bragg reflection mirrors (or DBR: Distributed Bragg Reflector) each composed of alternately formed two kinds of semiconductor layers of different refractive indexes are often used. When a DBR is formed with an AlGaAs group material, two kinds of semiconductor layers having different aluminum (Al) and gallium (Ga) compositions are used.
As described above, a surface-emitting type semiconductor laser has a structure in which an active layer is disposed within a resonator that is composed of an upper mirror and a lower mirror. Therefore, a current needs to be supplied to the active layer through DBRs. The DBR has a high resistance because of energy barriers at interfaces of the different kinds of formed semiconductor layers composing the DBR. Japanese Patent 2646799 describes a technology to reduce the resistance of a DBR by forming, between semiconductor layers composing the DBR, a composition graded layer (or graded index (GI) layer) whose composition is gradually changed.
Also, the surface-emitting type semiconductor laser has a current constricting layer formed near the active layer for controlling the current path of a current flowing through the active layer, and therefore is often formed into a mesa structure for forming the current constricting layer. In other words, the surface-emitting type semiconductor laser is often formed into a structure in a columnar configuration by etching the upper mirror, the active layer, and the lower mirror to an intermediate point thereof. An example of such a surface-emitting type semiconductor laser in a mesa structure is described in Japanese Laid-open Patent Application JP-A-2003-522421.
It is noted that, in the surface-emitting type semiconductor laser having a mesa structure described above, one of electrodes is formed on the upper mirror, and the other electrode is formed on the lower mirror that is exposed by etching. As a result, a current can be supplied to the active layer from an intermediate point of the lower mirror, and therefore the resistance can be reduced, compared to the case where a current is supplied to the active layer through the entire thickness of the lower mirror. The electrode formed on the lower mirror that is exposed by etching needs to have ohmic contact with the lower mirror.
The electrode may need to be formed on a lower composition layer with a smaller aluminum (Al) composition among the layers composing the above-described DBR that is formed with an AlGaAs group material, because it is difficult to have ohmic contact with a higher composition layer with a higher aluminum (Al) composition. For this reason, when the lower mirror is etched, a lower composition layer needs to be exposed. Etching of the upper mirror, the active layer, and the lower mirror to an intermediate point thereof are often conducted by dry etching. If a lower composition layer can be exposed by etching the lower mirror by dry etching, there would be no problem in forming the electrode.
On the other hand, if a higher composition layer is exposed by dry etching, the higher composition layer needs to be removed by wet etching with an etchant, such as, for example, hydrofluoric acid (HF), to thereby expose a lower composition layer. However, when a GI layer described above is formed in the lower mirror, and a higher composition layer is exposed at the surface by dry etching, if a lower composition layer is exposed by etching, a GI layer on the lower composition layer (wherein the GI layer has a greater aluminum (Al) composition than the aluminum (Al) composition of the lower composition layer) would remain without being etched. If an electrode is formed on the GI layer, sufficient ohmic contact cannot be obtained, which leads to a problem of higher resistance. Also, because the electrode formed on the GI layer has poor adhesion with the GI layer, there is a possibility that the electrode may be peeled off during the mounting process, and the yield may be lowered.